An organic electroluminescent device is generally composed of two electrodes and one or more organic material layers that are disposed between the electrodes. In detail, the organic light emitting device may have the same structure as the structure shown in FIG. 1. In FIG. 1, 1 represents a substrate, 2 represents a first electrode, 3 represents an organic material layer and 4 represents a second electrode. In an organic electroluminescent device having the above structure, if a voltage is applied between the first electrode and the second electrode, a hole is introduced from the first electrode and an electron is introduced from the second electrode into the organic material layer. An exciton is formed by recombining the hole and the electron. A photon corresponding to a difference of energy is emitted while the exciton falls to a bottom state. By this principle, an organic electroluminescent device emits visible rays, and an information display diode or illumination diode may be manufactured by using this.
In general, an organic electroluminescent device may be manufactured by a method for depositing a first electrode on a substrate, depositing one or more organic material layers, and depositing a second electrode. Accordingly, in order to emit light generated in an organic material layer, an electrode in a direction of emitting light should be transparent. When light is emitted in a first electrode direction, a substrate as well as a first electrode should be transparent.
Unlike a display, in the case of an illumination requiring a wide light emitting area, in general, a structure in which light is emitted in a first electrode direction has been developed. Since there is a limit in technology for increasing electric conductivity of the transparent electrode, the transparent electrode of the organic electroluminescent device for illumination requires a metal auxiliary electrode. The reason is that formation of the metal auxiliary electrode on the first electrode is technically easier than formation of the metal auxiliary electrode on the second electrode.
As described above, in the case of an organic electroluminescent device in which light is emitted in the first electrode direction, light emitted in the organic material layer passes through the first electrode and the substrate and is emitted to the air. The substrate is generally formed of glass in which a blocking property of air and moisture is excellent, and the glass has a refractive index of about 1.5. On the other hand, it is known that an average refractive index of the organic material layer emitting light is about 1.8.
In general, when light progresses from a medium having a high refractive index to a medium having a low refractive index, light that is incident at a critical angle or more does not pass through and is totally reflected. Accordingly, in the case of an organic electroluminescent device, since the refractive index of the organic material layer in which light is generated is about 1.8, only light that is incident at a critical angle or less passes through the glass substrate having the refractive index of about 1.5, such that a great loss of light occurs. This phenomenon occurs even when light is emitted in the glass substrate to the air having the refractive index of 1.
An example of a loss of light due to a difference of the refractive indexes between the transparent electrode and the glass substrate of an organic electroluminescent device is illustrated in FIG. 3.
Accordingly, as described above, an external light extraction for improving an efficiency of an organic electroluminescent device by preventing light from being emitted from a transparent substrate to an air layer totally reflected in the transparent substrate, and lost in the loss of light generated at interfaces having different refractive indexes has been studied.
For example, Korean Unexamined Patent Application Publication Nos. 10-2005-0001364 and 10-2005-0118566, and Japanese Unexamined Patent Application Publication Nos. 2007-005277, 2006-023683, 2005-174701, 2005-063926, and 2004-253199 disclose a method for changing the surface of a flat glass substrate in order to prevent a loss of light generated at an interface between the glass substrate and air.
Efficiency of an organic electroluminescent device may be improved by significantly recovering light lost by total reflection in the organic substrate by the above method. However, this method has a limit in that an effect is limited to only light transmitted to a glass substrate in light emitted in an organic material layer. In order to transmit all light emitted in an organic material layer to a glass substrate, a refractive index interface between an organic material layer and a glass substrate may be removed by using a glass substrate having the refractive index of 1.8 or more, but this method is not suitable to mass production.
Various studies for internal light extraction for extracting light confined by the organic material layer in conjunction with the study for the above external light extraction have been made.
For example, Japanese Unexamined Patent Application Publication No. 2006-269328, Korean Unexamined Patent Application Publication No. 10-2007-0030124 and Nature Photonics 2, 483-487 (2008) disclose various methods with respect to internal light extraction for extracting light confined by an organic material layer.
In the case of the illumination using an organic electroluminescent device, a transparent substrate should be used. In consideration of mass production, recently, the refractive index of a transparent substrate may not greatly exceed 1.52.
On the other hand, since the refractive index of a transparent electrode or a organic material layer is about 1.8, which is high, in order to extract light confined when light progresses at a progress angle of a critical or more in a transparent electrode and organic material layer, a method for forming a layer that can perform refraction of the progress angle between a transparent electrode and a transparent substrate may be used. As described above, the layer that can perform refraction of a progress path between a transparent electrode and a transparent substrate may receive a greater quantity of light emitted in an organic material layer as similarity of the refractive index thereof to the refractive indexes of the transparent electrode and the organic material layer is increased. When an average scattering angle is increased, a possibility of not total reflecting light but transmitting light when light passes through a transparent substrate layer may be increased.
Japanese Unexamined Patent Application Publication No. 2006-269328 discloses a method for planarizing an uneven structure by a material having a similar refractive index to a organic material layer after the unevenness is formed in order to refract light in the glass substrate. However, the formation of the uneven structure that can exhibit a sufficient scattering effect on the glass substrate like the above method is technically difficult and it is difficult to obtain a high refractive material that can planarize the uneven structure. In the case of a generally shaped organic material, a refractive index is not high, and in the case of an inorganic material, there is a method for performing polishing after the inorganic material is excessively deposited, but this method is not effective for mass production. Another problem is that even though a sufficient scattering property of the substrate on which the unevenness is formed is exhibited, in the case where a planarization layer is formed by materials in which a refractive index difference is not large, the scattering property is largely decreased by half, such that it is difficult to obtain an expected performance. Technologies disclosed in other prior arts have a possibility in terms of conception, but it is difficult to apply the technologies for mass production in practice.
In particular, in the case of an organic electroluminescent device, since a very large light emitting area is necessary as compared to a light emitting diode, there is a larger limit in a process that can perform mass production.